Organic photoconductors have been widely used in electrophotography such as laser printers and copiers to serve the function as an image transfer medium for image formation. There are several distinctly attractive features of using organic photoconductors compared to inorganic photoconductors; these include ease of manufacturing, low cost, non-toxicity, and flexibility of tailoring structures and properties. Infrared sensitive organic photoconductors have been found and used in printers that incorporate diode lasers as the light source. Function-separated photoconductors have been constructed to achieve optimal photoelectric response and provide acceptable wear resistance in repeated use. In dual-layer photoreceptors, charge generation and charge transporting layers are constructed to perform their respective functions. A subbing layer may be deposited between the conducting substrate and the charge generation layer to provide good adhesion in the junction and to block charge carriers which may be otherwise injected from the conducting substrate. In some instances, conductive particles at sub-micron size are embedded in the subbing layer to prevent reflection of light beams from the conducting substrate. To reinforce wear resistance, a thermosetting polymer film of high mechanical strength may be coated on the charge transporting layer to form a protective layer for the photoreceptors.
As a result of increasing demand on high-quality printing, versatile laser printers affording high-speed printing and good resolution are being constantly developed and commercialized. To fit the printing quality of various types of printers, it is important to choose suitable organic photoconductors exhibiting an optimal photosensitivity in association with toners to produce high-quality printing. In addition to providing the desirable photosensitivity, photoreceptors are also required to sustain high charge voltage in the dark. The ability to experience only a minimal dark decay of charging potential is an important prerequisite for photoreceptors to exhibit acceptable photoelectric response. Undesirable printed image patterns such as background in white and ghosting will be produced as a result of insufficient charging voltage.
There are several organic materials that are highly photosensitive to light in the wavelength range of 750 nm to 850 nm, corresponding to the emission of diode lasers. Among them, phthalocyanine, squaraines and perylenes are especially of interest because of their expedient photoresponse. Recently, much attention has been directed to the research and development of oxytitanium phthalocyanine for use as a charge generation component of organic photoconductors. It was shown that charge generation efficiency and related photoelectric properties of oxytitanium phthalocyanine depend closely on the crystal forms of the material. Charge generation efficiency close to unity has been reported for a certain crystal form known as Y-TiOPc.
It was disclosed in the teaching of U.S. Pat. No. 4,898,799 that Y-TiOPc can be obtained by the treatment of chlorine-containing solvents such as dichloroethane and dichlorobenzene following re-precipitation of the sulfuric acid solution of the material. U.S. Pat. Nos. 5,132,197 and 5,432,278 taught a highly photosensitive oxytitanium phthalocyanine (I-TiOPc) obtained by treating the material with n-butyl ether following re-precipitation of the sulfuric acid solution. The teachings of U.S. Pat. Nos. 5,298,617 and 5,440,029 disclosed hydrated oxytitanium phthalocyanine of which crystal structure could also be classified as the Y form. In a related U.S. Pat. No. 5, 567,559, the content of which is incorporated herein by reference, it was disclosed that a unique form of highly sensitive and ultra-pure oxytitanium phthalocyanine can be obtained by complexation-mediated crystal transformation incorporating ammonia gas and organic solvents as the transformation medium. The ammonia-modified oxytitanium phthalocyanine was characterized by a unique set of spectroscopic data, such as x-ray diffraction and optical absorption, spectrum, relative to Y-TiOPc or I-TiOPc. In line with the exploitation of oxytitanium phthalocyanine as a xerographic material, major efforts have been directed to the modification of crystal structure to achieve efficient charge generation and resulting high photosensitivity of oxytitanium phthalocyanine.
It is equally important to find methods for the effective dispersion of oxytitanium phthalocyanine in a polymer matrix so to form a high-quality charge generation layer. Homogeneous dispersion and environmental stability of the charge generation layer are key factors related to the so-called "cycle down" of dark development potential in xerographic processes. In the teaching of U.S. Pat. No. 5,384,222, it was disclosed that the dark decay for a charge generation layer containing Type IV (Y form) oxytitanium phthalocyanium could become a serious problem after repeated uses, and that judicious choices of polymer binders such as polystyrene-4-vinyl pyridine may provide a way to alleviate the problem of cycling down
Good dispersion and the resulting appreciable photosensitivity have been shown in U.S. Pat. No. 5,112,711 for a charge generation layer comprising a combination of oxytitanium phthalocyanine and, to a lesser extent, fluorine-substituted oxytitanium phthalocyanine. In U.S. Pat. No. 5,283,146, an amount of less than 10 parts by weight of nitro- or halogen-substituted oxytitanium phthalocyanine was added to 100 parts by weight of oxytitanium phthalocyanine to form the charge generation component exhibiting good dispersion and desirable photoelectric properties.
U.S. Pat. No. 5,153,313 taught a process involving the precipitation of photosensitive mixtures from organic acid solutions for the preparation of well-mixed composites consisted of a metal-free phthalocyanine, a metal phthalocyanine, and a metalloxy phthalocyanine. In U.S. Pat. No. 4,981,767, evaporated mixed crystals of phthalocyanine compounds, whose compositions included H2-phthalocyanine, Cu-phthalocyanine, TiO-phthalocyanine and VO-phthalocyanine etc., were disclosed to achieve the functions of increased heat and light stability and sufficient photosensitivity.
The aforementioned prior art teachings provided certain improvements in the photosensitivy of photoconductors; however, none of them discussed the issue relating to the variations in the photosensitivity as the composition of the photosensitive material changes. The disclosed art also never taught or suggested any method which can be utilized to tailor the photosensitivity in various photoreceptor applications. It is highly desirable to develop photoreceptors which can provide variable photosensitivity so as to be able to provide a wide range of compatibility with the various toners to achieve preferred printing quality. Furthermore, in the prior art teachings, the amount of nitro- or halogen-substituted derivative added to unsubstituted oxytitanium phthalocyanine was reported to be limited within 10 wt %.
In an article entitled "Cocrystalline Mixtures of Titanyl fluorophthalocyanine and Unsubstituted Titanyl Phthalocyanine," by M. F. Molaire, J. T. Henry, T. Zubil, and J. E. Kaeding, IS&Ts NIP13, International Conference on Digital Printing Technologies, it was reported that the crystalline mixtures of titanyl fluorophthalocyanine and unsubstituted titanyl phthalocyanine exhibit electrophotographic sensitivities that are at least 50% faster than the individual phthalocyanines treated in the same manner. This article examplifies that commonly observed synergism in a mixture of photosensitive compounds especially charge generation materials. While the synergism may be desired in that it can improve the electrophotographic sensitivities of individual photosensitive compounds, but too high a photosensitivity may not be desired in certain applications. In fact, different applications may require different levels of photosensitivity. In order to design a simple photosensitive system that can be advantageously and conveniently utilized in a wide range of different working conditions, it is highly desirable to explore such a photosensitive system which would exhibit tunable photosensitivity.